Industrial plants dealing with mining, production or storage of explosive or flammable gases and vapors such as hydrocarbons (methane, ethane, etc.), fuels of different kinds, hydrogen, acetylene, etc. are in constant danger of accidents. Explosions may cause fires, thus there is inherent danger from both the explosion itself and from the consequent ensuing fires. In addition, fires may result from a plethora of diverse causes, and when occurring in such plants, such fires may themselves cause explosions. The dangers are to both personnel and equipment, and the resulting damages may be in the worst cases loss of human lives and large financial losses to the owners of the plants.
Additionally, the release of the gases in question has a negative impact on the environment. As a result, regulatory laws have been introduced around the world to impose monitoring standards and heavy fines to companies that do not show due diligence in early detection of fires and prevention of inordinate releases of such materials.
The likelihood of explosions increases, up to a point, with increasing gas concentrations. Accordingly, over the past decades a large number of gas concentration measuring devices and fire detection instrumentation has been developed and used in mining, production and storage plants. Until recently only local detectors (for gases) or non-imaging IR and UV detectors (for flames) have been deployed. A gas detector of this type can easily miss the target gas if the gas cloud is present but does not physically meet the position of the detector (or path in case of cloud movement). This is due to the use of contact methods, such as chemical reactions with the gas. In the case of fire detection, the monitor is based on a single detector which does not provide an image of the field (i.e., scene) being monitored. Therefore, the monitor cannot provide the necessary information on the location and size of the fire.
Current industry instrumentation does not allow for the detection, identification, and location of the concentration, size and prognosis information of explosive gas or vapor clouds and flames due to incipient fires. Accordingly, current instrumentation cannot meet the additional requirements of being operable from a distance, in harsh environments, usually outdoors, and with minimal false alarms due to signals from other possible infrared sources, such as sun reflections, welding arcs, halogen lamps etc. The alarms provided by such detection instruments may be effectively used by the plant operators to prevent damages and losses of human lives through a number of possible actions. An example of such actions may be partial or total plant shut down, the request of fire department involvement, or other preventive or corrective action.
Furthermore, such infrared imaging devices can be used to quantitatively measure the radiance of each pixel of a scene only if the environment radiation changes (due mainly to environment temperature changes) contributing to the detector signals, can be monitored and corrected for. This is due to the fact that a quantitative measurement of infrared radiation from a scene is based on a mathematical relation between the detector signal and the radiation to be measured. This relation depends on the environment state during the measurement, and therefore the quantitative scene measurement can be done only if the environment state, and how the environment state affects that relation, is known during the measurement. The environment radiation sensed by the detector elements originates mainly from the optics and enclosures of the imaging device (besides the scene pixel to be monitored), and is a direct function of the environment temperature. If this radiation changes in time, it causes a drift in the signal, which changes its relation to the corresponding scene radiation to be measured and introduces inaccuracy.
This resulting inaccuracy prevents the use of such devices, especially in situations where they have to provide quantitative information on the gas to be monitored and have to be used unattended for monitoring purposes over extended periods of time, such as, for example, for the monitoring of a scene in industrial installations and facilities.
One known method for performing drift corrections is referred to as Non-Uniformity Correction (NUC). NUC corrects for detector electronic offset and partially corrects for detector case temperature drifts by the frequent use of an opening and closing shutter which is provided by the camera manufacturer. This NUC procedure is well known and widely employed in instruments based on microbolometer detectors. The shutter used for NUC is a moving part and therefore it is desirable to reduce the number of openings and closings of such a component when monitoring for gas leakages in large installations, requiring the instrument to be used twenty-four hours a day for several years without maintenance or recalibration. Frequent opening and closing of the shutter (which is usually done every few minutes or hours) requires high maintenance expenses.
To reduce the amount of shutter operations when using NUC techniques, methods for correcting for signal drift due to detector case temperature changes occurring between successive shutter openings have been developed by detector manufacturers, referred to as blind pixel methods. Known blind pixel methods rely on several elements of the detector array of the imaging device being exposed only to a blackbody radiation source placed in the detector case, and not to the scene radiation (i.e. being blind to the scene). However, such methods can only account and compensate for environmental temperature changes originating near and from the enclosure of the detector array itself, and not for changes originating near the optics or the enclosures of the imaging device. This is because in general there are gradients of temperature between the detector case and the rest of the optics and device enclosure. Therefore, known blind pixel methods may not satisfactorily compensate for environment radiation changes in imaging devices with large and/or complex optics, such as, for example, optics having an intermediate focal plane requiring at least two optical lenses, and optics having reflective and/or refractive surfaces for directing radiation in part through an optical lens towards the detector and/or in part directly towards the detector, as will be described below.